1. Field of the Invention
The present invention relates to a wireless communication device, a transmission power controlling method, and a program that control the power for transmitting a radio signal.
2. Description of the Related Art
As a technique for providing efficient wireless communications, a procedure for dynamically acquiring an unused channel by frequency hopping with a plurality of channels is implemented in a large number of wireless communication systems.
In addition, examples of wireless communication devices including a wireless LAN (Local Area Network) communication function to which a standard such as IEEE 802.11 is applied include a device that can change a value of transmission power for transmitting a radio signal from the wireless communication device by setting parameters from the outside. In such a wireless communication device, in order to reduce radio wave interference between wireless communication devices and to prevent wire tapping due to a radio wave leak, a value of transmission power that does not exceed a value determined as an upper limit standard may be adjusted.
In addition, as forms of wireless communication, a single-hop scheme in which a transmitting node and a receiving node directly communicate with each other and a multihop scheme in which a transmitting node communicates with a receiving node via a relay node are known.
Examples of multihop schemes include a variety of schemes such as the AODV (Ad-hoc On Demand Distance Vector) and the OLSR (Optimized Link State Routing) protocols discussed in the MANET-WG (Mobile Ad-hoc Network Working Group). In any one of the schemes, routing control information is exchanged between peripheral nodes to construct a communication path.
However, such a technique has problems described below.
A first problem is that because wireless communications limit the number of available channels, wireless networks for a plurality of applications coexist in the same area. Therefore, a large number of wireless communication devices exist in the same area. Disadvantageously, an increased amount of communications between these wireless communication devices may cause congestion and longer delay time.
The reason is as follows. In wireless communications, to avoid effects due to the distance of wireless transmission and obstacles on a wireless transmission path, communication is performed in accordance with maximum transmission power defined in a standard. Therefore, it is highly likely that a radio wave teaches an area unnecessary for the communication. If a radio wave reaches an area unnecessary for the communication, the number of collisions of signals on a transmission path, signal retransmission due to the collisions, and transmission waiting time due to carrier detection on the transmission path increase, resulting in degraded throughput of an overall network.
A second problem is the fact that transmitted signals, that can be received even outside an area that the signals need to reach, may lead to security concerns such as wire tapping.
The reason is as follows. An appropriate value as a transmission power value cannot be known in advance, so that the maximum transmission power defined in a standard has to be set to provide stable communications.
Thus, techniques for, based on radio wave strength and the noise level of a received radio signal, controlling the transmission power for transmitting a radio signal have been conceived (For example, see JP2010-200164A and JP2010-016561A.).
However, the technique described in JP2010-200164A has a problem that two radio systems need to be used.
In addition, the techniques described in JP2010-200164A and JP2010-016561A do not take into account the number of wireless communication devices with which a wireless communication device that transmits a radio signal communicates. Therefore, even if the number of wireless communication devices with which the wireless communication device communicates varies, the same transmission power value is calculated. Thus, for example, when a calculated transmission power value is the best value for transmission to one wireless communication device, if the wireless communication device attempts to communicate with a plurality of wireless communication devices, the radio wave may not reach the device. In addition, when a calculated transmission power value is the best value for transmission to a plurality of wireless communication devices, if the wireless communication device transmits a signal to communicate with only one wireless communication device, the problems described above (e.g., deteriorated throughput) may arise.
An object of the present invention is to provide a wireless communication device, a transmission power controlling method, and a program that solve these problems.